The invention relates to an RF surface resonator (RF coil) for transmitting and/or receiving circularly polarized electromagnetic waves, notably for magnetic resonance imaging apparatus (MRI apparatus) in which a basic magnetic field (vertical field) that extends perpendicularly through an object to be examined and a circularly polarized RF field are generated, and also relates to an MR apparatus that is provided with such an RF surface resonator.
In MRI apparatus of this kind the basic magnetic field is generated generally between two pole plates wherebetween an examination zone, for example for a patient, is situated. The basic magnetic field extends through a patient essentially in a direction perpendicular to the longitudinal axis thereof (generally speaking, vertically) so that, as opposed to imaging apparatus with a tubular examination space in which the basic magnetic field extends in the direction of the longitudinal axis of the patient, the patient remains suitably accessible from practically all sides, that is, also during image acquisition; in particular it is also possible to carry out interventional examinations. Systems of this kind, therefore, are also referred to as open MR systems or open MR imaging apparatus.
In order to generate an adequately strong and homogeneous basic magnetic field it is advantageous to keep the distance between the pole plates as small as possible. On the other hand, in order to avoid deterioration of the accessibility of the examination zone that is situated between the pole plates and of the patient comfort, an as large as possible distance is required; however, such a large distance may give rise to an inhomogeneous field. Therefore, as flat as possible (or at least flattish) RF conductor structures (surface resonators) are used to generate the RF field as well as to detect MR relaxation events; these structures are arranged over the pole plates and as near as possible thereto (or on an RF shield).
A surface resonator of this kind is known from U.S. Pat. No. 5,153,517. This resonator is composed of two sub-systems, each of which is formed by two flat current loops and is arranged in such a manner that these systems are situated in a plane perpendicular to the direction of the basic magnetic field and are capable of generating a circularly polarized RF field. RF coils of this kind offer the advantage that in comparison with other configurations they require only a comparatively small transmission power, because essentially only field components that are relevant to the excitation of the nuclear magnetization or the detection of the relaxation events are generated or detected (that is, circular field components).
However, a problem is encountered in the proportioning and the application of such RF surface resonators, notably in the case of high field strengths of the main magnetic field; this problem concerns the radiation characteristic of the RF field that extends perpendicularly to the resonator plane. On the one hand an as homogeneous as possible field is required, that is a constant radial field strength distribution within the examination zone, whereas on the other hand it is desirable that the field decreases comparatively steeply in the space that is situated radially outside said examination zone; this is desirable so as to avoid backfolding or aliasing artifacts which may be due to the fact that the distribution of the main magnetic field is no longer homogeneous at that area and also due to the presence of attendant staff near the examination zone.
It is an object of the invention, therefore, to provide an RF surface resonator that is intended notably for use in an open magnetic resonance imaging apparatus of the kind set forth and is capable of generating an RF field that has a high field strength which is homogeneous (meaning that it is essentially constant or at least that it varies only gradually without gradients in the field strength that influence the image quality) within an examination zone but decreases comparatively steeply in a space that is situated radially outside said zone.
This object is achieved by means of an RF surface resonator of the kind set forth which, in conformity with claim 1, is characterized in that there are provided a plurality of conductor structures that extend in radial directions from a central zone as well as at least one conductor loop that encloses the central zone and serves as a return for the current of the conductor structures, the radial distance between said conductor loop and the central zone being chosen to be such that a desired radiation characteristic is achieved in a plane extending perpendicularly to the surface resonator.
It is a special advantage of this solution that the radiation characteristic of the resonator can be comparatively simply adjusted (also in the built-in condition) by fitting additional conductor loops of the kind set forth.
The dependent claims relate to advantageous further embodiments of the invention.
In the embodiment in conformity with claim 2 a conductor density (number of conductor segments per unit of surface area) can be realized that remains essentially the same in the radial direction, so that a correspondingly more homogeneous variation of the radiation characteristic is obtained in the radial direction.
The embodiment disclosed in claim 3 enables a radiation characteristic which differs in different perpendicular planes to be achieved in a simple manner when, for example the RF field is intended to traverse exclusively an elongate object.
The characteristics disclosed in the claims 4 to 8 enable further influencing of the current distribution across the surface resonator in various ways so as to achieve a desired variation of the radiation characteristic.
In the embodiments that are disclosed in the claims 9 and 10 the radiation characteristic can be effectively influenced by electrical separation of parts of the surface resonator or disturbances of the RF field that are due to external interventions in the examination zone can be compensated.